Contamination monitoring and control techniques for use with an optical metrology instrument

ABSTRACT

A technique is provided for monitoring and controlling surface contaminants on optical elements contained within the optical path (or sub-path) of an optical metrology instrument. The technique may be utilized in one embodiment in such a manner as to not require that additional components and/or instrumentation be coupled to, or integrated into, existing metrology equipment. Surface contaminants on optical elements within an optical metrology instrument are monitored so that cleaning procedures can be performed as deemed necessary. The cleaning procedures may include the use of exposing the optical elements to optical radiation. The optical metrology instrument may be an instrument which operates at wavelengths that include vacuum ultra-violet (VUV) wavelengths.

RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 60/795,467 filed Apr. 27, 2006, the disclosure of whichis expressly incorporated herein by reference. The present applicationalso expressly incorporates by reference the following two U.S. patentapplications concurrently filed on the same date as the presentapplication: U.S. patent application Ser. No. ______, entitledCONTAMINATION MONITORING AND CONTROL TECHNIQUES FOR USE WITH AN OPTICALMETROLOGY INSTRUMENT by Harrison and Weldon; and U.S. patent applicationSer. No. ______, entitled CONTAMINATION MONITORING AND CONTROLTECHNIQUES FOR USE WITH AN OPTICAL METROLOGY INSTRUMENT by Harrison andWeldon.

BACKGROUND OF THE INVENTION

The present application relates to the field of optical metrology andmore particularly to optical metrology that may be performed in thevacuum ultraviolet (VUV).

In one embodiment, a means by which accurate and repeatable opticalmetrology may be performed in the vacuum ultraviolet (VUV) is provided.In one embodiment, the technique disclosed herein can be used to ensurethat vacuum ultraviolet reflectometers generate highly stable andrepeatable results in the presence of both gaseous and surfacecontaminants. In another embodiment, the techniques disclosed hereinprovide a means for obtaining accurate reflectance data from sampleswhose surfaces themselves may be contaminated.

Optical metrology techniques have long been employed in process controlapplications in the semiconductor manufacturing industry due to theirnon-contact, non-destructive and generally high-throughput nature. Thevast majority of these tools operate in some portion of the spectralregion spanning the deep ultraviolet to near-infrared wavelengths(DUV-NIR generally 200-1000 nm). The continuous push towards developingsmaller devices comprised of thinner layers has challenged thesensitivity of such instrumentation. An effort to develop opticalmetrology equipment utilizing shorter wavelengths (below 200 nm), wheregreater sensitivity to subtle changes in processing conditions can berealized has been considered. Approaches to performing opticalmeasurements at shorter wavelengths such as a system and method for avacuum ultraviolet (VUV) reflectometer are described in U.S. applicationSer. No. 10/668,642, filed on Sep. 23, 2003, now U.S. Pat. No. 7,067,818and U.S. application Ser. No. 10/909,126, filed on Jul. 30, 2004, nowU.S. Pat. No. 7,126,131 the disclosures of which are both expresslyincorporated herein by reference.

Contamination of optical surfaces like windows and mirrors is a seriousimpediment to the operation of optical instruments in the VUV. Moistureand residual molecules, particularly hydrocarbon compounds, may depositon such surfaces over time dramatically reducing their performance.These effects have formed the focus of previous investigations owing totheir impact on the design, development and performance of 193 and 157nm lithographic exposure tools.

It order to ensure the tremendous sensitivity enhancements theoreticaloffered by VUV optical metrology instruments are practically realized,it would be highly desirable to develop an instrument with the inherentcapability of reducing, removing or altogether eliminating the build upof contaminates on its optical surfaces. Furthermore, if thisself-cleaning capability could be realized without the addition ofpotentially expensive and complicated components it would represent agreat benefit to tool owners.

When present on the surfaces of samples under investigation, contaminatelayers may significantly contribute to measured optical responses in theVUV yielding inaccurate and/or erroneous results. These effect are ofparticular concern when the samples are comprised of ultra thin films(<100 Å), whose thicknesses may themselves be comparable to thethicknesses of the contaminate layers.

One technique contemplated for improving the measurement ofsemiconductor wafers by removing contamination layers in a cleaning stepincludes employing microwave radiation and/or radiant heating, prior tomeasurement. Although enhanced measurement repeatability is reportedusing this approach, the method requires that a separate cleaning systembe coupled to an existing measurement system resulting in increasedsystem cost and design complexity.

In light of these disadvantages it would be desirable to develop ameasurement system that was itself capable of removing contaminants fromthe surface of samples, so as to ensure accurate and highly repeatableresults were achieved. Such an instrument would be capable ofsimultaneously cleaning and measuring specific locations on the samplewithout requiring additional components, above and beyond those normallyrequired for measurement, thereby reducing system cost and designcomplexity. Furthermore, such an instrument would not require alignmentof separate cleaning and measurement subsystems. In addition, such aninstrument would avoid needlessly “cleaning” the entire sample, while atthe same time ensuring that consistent cleaning results were obtained atall measurement locations.

SUMMARY OF THE INVENTION

One embodiment of the disclosed techniques provides a technique forgenerating and subsequently monitoring the controlled environment(s)within a VUV optical metrology instrument in such a manner as tominimize, or all together eliminate, the build-up of contaminants on thesurfaces of optical elements that may result in performance degradation.

Another embodiment discloses a technique for reducing surfacecontaminants from optical elements contained within the optical path (orsub-path) of an optical metrology instrument. The technique may beutilized in one embodiment in such a manner as to not require thatadditional components and/or instrumentation be coupled to, orintegrated into, existing metrology equipment.

Another embodiment discloses a technique whereby surface contaminants onoptical elements within an optical metrology instrument are monitored sothat cleaning procedures can be performed as deemed necessary. Thetechnique may further enable separate optical paths of the instrument tobe monitored, and subsequently cleaned, independent of one another.

In yet another embodiment, a technique is disclosed for removingcontaminants from the surface of a sample prior to recording an opticalresponse from said sample in order to ensure that accurate results areobtained. In one alternative, the technique may be implemented in such amanner as to not require that additional components and/orinstrumentation be coupled to, or integrated into, existing metrologyequipment.

In yet another embodiment, a technique is disclosed for characterizingcontaminants on the surface of a sample. In addition to providinginsight into the nature of the contaminant itself, the technique alsoprovides a means by which accurate sample measurements can be performedin light of contamination layers which may be present on their surfaces.

In another embodiment, a method of monitoring surface contaminant levelson optical elements within an optical metrology tool is provided thatmay include performing a plurality of intensity measurements andanalyzing the intensity measurements of at least two of said pluralityof measurements. The method may further include determining thestability of the surface contaminant levels from a comparison ofintensity measurements of the at least two of said plurality ofmeasurements.

In still another embodiment, a method of monitoring surface contaminantlevels on optical elements within an optical metrology tool thatutilizes at least some wavelengths less than deep ultra-violet (DUV)wavelengths is provided. The method may comprise providing a referenceoptical path and a sample optical path, the reference optical path andthe sample optical path being optically balanced and performing aplurality of intensity measurements utilizing at least some wavelengthsless than DUV wavelengths. The method may further comprise analyzing theintensity measurements of at least two of said plurality of measurementsand determining the stability of the surface contaminant levels based atleast on part on the analysis of the intensity measurements of the atleast two of said plurality of measurements. The intensity measurementsmay be performed utilizing at least one of the reference optical path orthe sample optical path.

In another embodiment, a method of cleaning surface contaminants onoptical elements within an optical metrology tool is provided thatcomprises performing a plurality of intensity measurements wherein theintensity measurements expose the optical elements to radiation underconditions suitable for removing the surface contaminants. The methodmay further comprise analyzing the measured intensity of at least two ofsaid plurality of measurements and determining whether surface cleaningof the optical elements is desirable based upon the analyzing of themeasured intensities. If surface cleaning is determined to be desirable,surface cleaning may be performed by exposing the optical elements toadditional radiation.

A further understanding of the nature of the advantages of the conceptsdisclosed herein may be realized following review of the followingdescriptions and associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, in which like referencenumbers indicate like features. It is to be noted, however, that theaccompanying drawings illustrate only exemplary embodiments of theinvention and are therefore not to be considered limiting of its scope,for the invention may admit to other equally effective embodiments.

FIG. 1 is an exemplary environmental monitoring flowchart.

FIG. 2 illustrates exemplary environmental monitoring results using abroad-band VUV data set.

FIG. 3 illustrates exemplary environmental monitoring results using aselect wavelength VUV data set.

FIG. 4 illustrates exemplary reflectance data obtained from clean andcontaminated silicon surfaces.

FIG. 5( a) illustrates an optical surface with contaminate layer.

FIG. 5( b) illustrates VUV exposure in an oxygen-containing ambientresulting in cleaning.

FIG. 5( c) illustrates a clean optical surface following treatment.

FIG. 6 illustrates reversible deposition and etch of one contaminantspecie and irreversible deposition of a second contaminant specieresulting from VUV irradiation in the presence of an ambient containingboth contaminants and oxygen.

FIG. 7 is an exemplary system contaminant monitoring flowchart.

FIG. 8 illustrates exemplary contaminate monitoring data using a selectVUV wavelength.

FIG. 9 is an exemplary contaminated sample measurement flowchart.

FIG. 10 illustrates an exemplary cleaning response profile for threedifferent contaminate films.

FIG. 11 illustrates an exemplary schematic representation of a VUVreflectometer.

FIG. 12 illustrates a broad-band referencing reflectometer coveringthree spectral regions including the VUV.

FIG. 13 is an exemplary operational flowchart for a VUV opticalmetrology instrument.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To enhance the sensitivity of optical metrology equipment forchallenging applications it is desirable to extend the range ofwavelengths over which such measurements are performed. Specifically, itis advantageous to utilize shorter wavelength (higher energy) photonsextending into, and beyond, the region of the electromagnetic spectrumreferred to as the vacuum ultra-violet (VUV). Vacuum ultra-violet (VUV)wavelengths are generally considered to be wavelengths less than deepultra-violet (DUV) wavelengths, i.e. less than about 190 nm. While thereis no universal cutoff for the bottom end of the VUV range, some in thefield may consider VUV to terminate and an extreme ultra-violet (EUV)range to begin (for example some may define wavelengths less than 100 nmas EUV). Though the principles described herein may be applicable towavelengths above 100 nm, such principles are generally also applicableto wavelengths below 100 nm. Thus, as used herein it will be recognizedthat the term VUV is meant to indicate wavelengths generally less thanabout 190 nm however VUV is not meant to exclude lower wavelengths. Thusas described herein VUV is generally meant to encompass wavelengthsgenerally less than about 190 nm without a low end wavelength exclusion.Furthermore, low end VUV may be construed generally as wavelengths belowabout 140 nm.

It is generally true that virtually all forms of matter (solids, liquidsand gases) exhibit increasingly strong optical absorptioncharacteristics at VUV wavelengths. In part, it is this ratherfundamental property of matter which is itself responsible for theincreased sensitivity available to VUV optical metrology techniques.This follows as small changes in process conditions, producingundetectable changes in the optical behavior of materials at longerwavelengths, can induce substantial and easily detectable changes in themeasurable characteristics of such materials at VUV wavelengths. Thishighly non-linear dependence of photon absorption cross section onwavelength presents tremendous opportunities for VUV optical metrologyinstrumentation; unfortunately it also introduces associatedcomplications.

One such complication relates to the fact that VUV radiation can notpropagate through standard atmospheric conditions. VUV photons arestrongly absorbed by O₂ and H₂O and hence, these species must bemaintained at sufficiently low levels (for example typically sub-PPM) soas to permit transmission along the optical path of metrologyinstrumentation. To this end vacuum or purge methods, using anon-absorbing purge gas (like nitrogen, argon or helium), are typicallyemployed. Purge methods can be reasonably effective at lowering theconcentration of gaseous O₂ and H₂O, but it is very difficult to removeadsorbed water in a timely fashion without employing vacuum techniques.Vacuum methods, though more efficient at removing adsorbed H₂O,inadvertently promote surface contaminant migration as a consequence ofthe increase in mean free path experienced by such species at reducedpressure.

In light of these disparate considerations, it follows that neithervacuum nor purge methods alone constitute the most effective means ofgenerating, and subsequently maintaining, the controlled environmentrequired to perform optical measurements in the VUV. Rather, it isdesirable to utilize a procedure which combines elements of bothtechniques in order to ensure optimum tool performance. In short, vacuumis initially employed to quickly lower the concentration of moisture andoxygen species to acceptable levels, following which the pressure withinthe instrument is back filled with a non-absorbing gas.

In one embodiment, in order to lower the concentration of adsorbedmoisture to an acceptable level it is typically desirable to lower thepressure in the tool to somewhere in the vicinity of 1×10⁻⁵−10⁻⁶ Torr.Care may be taken to ensure the optical surfaces of the instrument arenot exposed to VUV radiation during this time since this may lead to theformation of photo-deposited contaminant layers. This situation isreadily avoided by either shuttering or powering off VUV sources priorto evacuation.

The time required to achieve the requisite vacuum condition depends onmany aspects of the system (i.e. temperature, internal surface area ofinstrumental volume, pumping speed of vacuum system, etc.) but will belargely driven by exposure of internal surfaces to ambient air duringthe sample load process. As such, it follows that system pump-down timewill be minimized through intelligent use of load-lock mechanisms;thereby increasing system throughput and reducing the migration ofcontaminates.

The pump-down cycle time can also be shortened by applying energy to theadsorbed water bed through mechanical, thermal or radiative methods.Mechanical energy can be applied during the initial portion of the pumpdown cycle by bleeding in controlled purge gas. Thermal energy can beapplied by heating the walls of the instrument, however this approachcould promote contaminate migration and introduce mechanicalinstabilities. UV lamps can also be used to transfer energy directly toadsorbed water molecules, but may simultaneously result in contaminantphoto-deposition.

Once the concentration of absorbing species within the instrumentalvolume is sufficiently lowered, the controlled environment required tosupport operation (i.e. permit sufficient transmission of VUV photons)is achieved by back filling the volume of the instrument with ahigh-purity non-absorbing gas. While contamination considerations mayencourage maintaining the pressure of the instrument at elevated levels,mechanical considerations generally limit practical operating pressuresto somewhere in the neighborhood of atmospheric conditions.Consequently, pressures in the range of 300-700 Torr are typicallyemployed. Thus optical performance may be improved by increasing opticaltransmission. The optical transmission may be increased by lowering theoxygen and/or moisture content through the use of vacuum techniques. Inaddition, optical performance may also be increased by suppressing themigration of absorbing species (i.e. contaminants) from surfaces throughthe use of backfill techniques. Adsorbing contaminants which migratefrom instrument surfaces and adhere to optical surfaces cansignificantly degrade the performance of such elements, resulting inmirrors with reduced reflectivity and windows with decreasedtransmission characteristics.

Over time the quality of the controlled environment within theinstrument can be expected to degrade due to a variety of sources;including but not limited to outgassing from interior surfaces,permeation through materials, infiltration via leaks (both real andvirtual), and outgassing from samples themselves. Any of thesemechanisms can result in an increase in the concentration of absorbingspecies within the instrument and thus, a corresponding decrease in theoptical throughput of the system. It follows that it may be advantageousto monitor the state of the environment so that appropriate steps may betaken to restore it as required; thereby ensuring that measurementaccuracy is not compromised.

While many stand-alone methods, systems and sensors exist for monitoringthe quality of an enclosed environment, the most direct and arguablymost useful approach is to utilize the optical elements of the metrologyinstrument, in combination with a reference sample, to track opticalthroughput. The flowchart 100 in FIG. 1 illustrates how environmentalmonitoring may be achieved in this manner. First the volume of theinstrument is evacuated to a pre-determined pressure P_(L) using anappropriate vacuum system at step 110. Next, the instrument isbackfilled with a non-absorbing gas to a pre-determined measurementpressure P_(H) at step 120. Once the measurement pressure has beenattained the intensity spectrum from the reference sample is immediatelyrecorded at time t₁ as shown at step 130. The concentrations ofabsorbing species are presumed to be at their lowest achievable level atthis point in time. Measurements on test samples are then performed atstep 140 for some pre-determined time period, following which theintensity spectrum from the reference sample is again collected at timet₂ as shown at step 150. As shown at steps 160 and 170, the ratio of thetwo intensity spectra (time t₁ and time t₂) from the reference sample isthen calculated and analyzed in order to determine the concentrations ofabsorbing species. At step 180, the determined concentrations are thencompared to user-defined threshold values in order to determine whetheror not the environment in the instrument is suitable to support furthermeasurements. As indicated at step 190, if the environment is suitable,then measurements on test samples are again conducted for anotherpre-determined time period by returning control to step 140. Conversely,if the environment is no longer adequate (as indicated by step 195) thenthe environment is regenerated by re-initiating the evacuation/backfillprocedure at step 110.

FIG. 2 presents ratios of reference measurements collected in thepresence of an atmosphere of non-absorbing gas containing traceconcentrations (1, 5, 10 and 20 PPM as indicated by plots 200, 205, 210,and 215 respectively) of both oxygen and water, to those collected inthe presence of pure non-absorbing gas. As is evident from the figurethe transmission through the controlled environment is considerablyreduced at wavelengths below 190 nm as the concentrations of oxygen andwater are increased. With a priori knowledge of the optical path length,instrumental pressure and absorption cross-section for oxygen and water,the concentrations of these species can be readily determined throughanalysis.

Alternatively, the ratio of reference measurements at discretewavelengths can also be used to provide a simple monitor of the qualityof the controlled environment. In FIG. 3 the ratio of referencemeasurements performed in the presence of non-absorbing gas containingtrace amounts of oxygen and water to those collected in the presence ofpure non-absorbing gas are plotted as a function of oxygen and waterconcentration for VUV wavelengths 124.6 nm, 144.98 nm, and 177.12 nm(shown as plots 300, 305 and 310 respectively). In practice the measuredtransmission values can be compared to user-defined thresholds in orderto determine the state of the controlled environment. The actualwavelengths used in the comparison can be chosen based on the absorptioncross-section of the absorbing species of interest.

The environmental monitoring procedures described herein presume thespectral intensity of the VUV source does not change appreciably betweenthe initial and final reference measurement times. While this may be areasonable assumption in many instances, it is noted that the spectralintensity of the source could be independently monitored in order toaccount for intensity fluctuations in situations where significantvariation was expected.

The time period over which such an instrument can be operated (andsample measurements can be reliably conducted) before requiringregeneration of the controlled environment could vary significantlydepending on the design of the instrument and on the manner in which itis operated. For well-designed (i.e. leak-tight) systems operated so asto minimize exposure of internal surfaces (i.e. where samples areintroduced via load-lock mechanisms) changes in the controlledenvironment will typically occur on a time scale considerably longerthan that required to measure a given sample. Hence, it will often bepossible to measure many such samples before requiring the controlledenvironment be regenerated. In any event, the reference measurementinterval can be adjusted as required such that short intervals can beemployed in cases where the environment is less stable and longerintervals can be used where the environment is more stable.

The controlled “measurement” environment established in the processoutlined in the flowchart of FIG. 1 may be created by backfilling theinstrument volume to a pre-determined pressure. Once this state isachieved the flow of purge gas to the instrument may be discontinued. Analternate method of operating such an instrument could be to equip theinstrument with a purge valve set to a specific “relief” pressure sothat purge gas could flow continuously through the instrument. Inprinciple this could lessen or altogether eliminate the need to“regenerate” the measurement environment since the build-up of absorbingspecies like oxygen and water could be limited. Implementation of thisapproach is difficult in practice owing to the high flow rates requiredto maintain sufficiently low concentrations of absorbing species (owingto back-streaming of contaminants through purge exhaust). Furthermore,the continuous purge may induce pressure fluctuations that couldadversely affect measurement stability.

A second difficulty associated with the use of VUV photons in opticalmetrology instrumentation relates to issues regarding surfacecontamination. Thin contaminate layers, which may only marginally affectthe performance of optical surfaces at longer wavelengths, maysignificantly degrade the response of such elements at VUV wavelengths.In addition to adsorbed layers, which may be expected to readily form onoptical surfaces under normal atmospheric conditions, organic andsilicone-based films may also be unintentionally photo-deposited on suchsurfaces when irradiated with VUV photons in the presence of acontaminate-containing ambient.

An example of the effect that contaminate layers may have on the VUVresponse of optical surfaces is provided in FIG. 4, wherein thereflectance of a “clean” silicon surface is compared to that of“slightly contaminated” and “more contaminated” surfaces as shown byplots 400, 405 and 410 respectively. As is evident from the figure thereflectance of the Si surface in the VUV region is significantlydegraded as function of contaminate accumulation. As photons in opticalmetrology tools typically encounter many such surfaces as they travelfrom source to sample and finally to detector, it follows that evensmall reductions in the optical performance of each surface canseriously impact the overall optical throughput of the instrument.

Fortunately it is in many cases, possible to reduce, remove oraltogether eliminate the build up of such contaminates on opticalsurfaces non-destructively through VUV irradiation in the presence of anambient containing trace concentrations of oxygen. When exposed to VUVwavelengths diatomic oxygen is dissociated into atomic oxygen, whichthen reacts with diatomic oxygen to form ozone. Both atomic oxygen andozone are highly reactive and capable of oxidizing surface contaminantsforming gaseous products, which may then be liberated.

This photo-etch cleaning process is illustrated schematically in FIG. 5.In FIG. 5A a contaminated optical surface 500 is shown havingcontaminates 505. In FIG. 5B the contaminated optical surface 500 isexposed to VUV radiation 510 in the presence of an oxygen-containingambient resulting in the removal of contaminates 505 from the surfacethrough the reactions outlined above. In FIG. 5C the resultant “clean”optical surface 520 is presented.

With certain contaminants (for example halogenated organic compounds andorganosilicones) the photo deposition reactions may be irreversible andhence, it may not be possible to fully remove them through photo-etchprocesses. In such instances irradiation of surfaces with VUV photons inthe presence of an ambient consisting largely of oxygen, but with eventrace levels of contaminate compounds, may only result in continuedgrowth of the contaminate layer.

In a more general case a surface may be exposed to VUV radiation in thepresence of an ambient containing oxygen, contaminates conducive to bothphoto-deposit and photo-etch (i.e. those formed via reversiblereactions) and contaminates which photo-deposit, but can not bephoto-etched (i.e. those formed via irreversible reactions). In suchcircumstances there could be at least three distinct processes takingplace; reversible deposition processes, irreversible depositionprocesses and back-reaction processes via etching. These three processesare depicted graphically in FIG. 6 by the hollow circles 610 and filledcircles 620 moving towards the surface (reversible and irreversibledeposition respectively) and by the hollow circles 610 moving away fromthe surface (etching), respectively. The relative rates associated withthese processes will depend on a variety of factors including thesurface concentration of adsorbed contaminants, the oxygenconcentration, the absorption cross section of said contaminants and theassociated VUV photon flux.

As oxygen may play a crucial role in the photo-etch process it followsthat it may be beneficial to monitor and control the concentration ofoxygen contained within the instrumental volume. In this manner, tracelevels of oxygen (or clean dry air) could be intentionally added to thenon-absorbing gas of the backfill so as to promote the cleaningprocesses without significantly compromising VUV photon flux formeasurement purposes. Depending on the relative rates of contaminationand etch it may be possible to operate VUV optical instrumentation insuch a manner as to not facilitate the build-up of undesirable materialsto begin with (i.e. where etch rates exceed contamination rates). Incases where significant quantities of contaminants are already presenton optical surfaces, the cleaning time required to remove such films maybe greatly reduced by temporarily increasing the concentration of oxygenabove the levels normally employed for data acquisition.

The concentration of trace quantities of oxygen intentionally added tothe controlled environment of the instrument could be monitored just asthe unintentional accumulation of oxygen and moisture (due to leaks,etc.) was tracked using the environmental monitoring methodology ofFIG. 1. Trace quantities of gases like oxygen could be accurately addedto the volume of the instrument using a mass flow controller. Practicalimplementation could be achieved by modifying the methodology of FIG. 1such that a fixed quantity of oxygen was added to the volume of theinstrument immediately prior to the commencement of sample testing. Toverify that the appropriate quantity of oxygen was added to the system,the intensity spectrum from the reference sample could be recorded andcompared to that obtained immediately following backfill with thenon-absorbing gas. Thus, for example, trace quantities of oxygen forexample in the range of 1 ppm or less and more preferably in the rangeof 0.1 ppm or less can be added to an controlled environment in orderaccelerate various cleaning mechanisms. In one embodiment the controlledenvironment may be at a sub-atmospheric pressure.

In principal, all optical surfaces in VUV metrology instruments aresusceptible to contamination effects. This includes not only opticalelements (i.e. windows, beam splitters, mirrors, etc), but also thesurfaces of samples themselves. As the concentration of contaminants inmetrology instruments may be expected to vary considerably with changesin the tool ambient, and through the introduction of samples, it followsthat in order to achieve optimum system performance it may be beneficialto monitor the accumulation (or removal) of said contaminants over timeso that appropriate cleaning measures may be taken.

If such surfaces are to be effectively cleaned via photo-etchingprocesses it follows that the VUV relative flux profile received byoptical surfaces during cleaning should closely match that receivedduring the initial photo-deposition process. Consequently, to achieveoptimum cleaning results it may be desirable to precisely configure andalign VUV cleaning systems with VUV optical metrology instruments insuch a manner as to ensure the VUV flux profiles received by opticalsurfaces are closely matched.

Accordingly it may be advantageous if VUV cleaning capabilities could beintegrated in to optical metrology instrumentation in such a manner asto not require their precise configuration and alignment. Furthermore,if such capabilities could be incorporated in a means that required veryfew additional components, above and beyond those already present in theoptical metrology tool, system design and cost requirement could begreatly reduced. An innovative means of accomplishing this is to utilizethe optical elements of the metrology instrument itself, in combinationwith a reference sample, to track the contamination state of the system.

The flowchart 700 in FIG. 7 illustrates how contaminant monitoring maybe achieved in this manner. First a series of n monitoring measurementsof the reference sample intensity are performed on the reference sampleand recorded as shown in step 710. Each measurement exposes the opticalpath of the instrument (and each of the optical elements encountered enroute) to a certain flux of VUV radiation for a known time interval.Hence, each measurement may be considered to impart a specific dose ofVUV radiation upon the optical surfaces in the instrument. Next, thereference sample intensity may be analyzed as a function of themeasurement number n. For example, the reference sample intensity may beplotted as a function of the measurement number n as shown in step 720.Thus, by recording the intensity at the detector as a function of thenumber of measurements it is possible to effectively track the opticalthroughput of the system as a function of VUV exposure dose. In oneembodiment, the number of measurements may be ten or less. If theenvironment of the system is sufficiently controlled, the number ofmeasurements may only be two.

Following these reference measurements the recorded results can beanalyzed to determine whether or not the cleaning process is completeand hence the optical throughput of the tool stable. For example, asshown in step 730, the plot of the reference sample intensity as afunction of measurement number n may be analyzed to determine thecontamination state, and if unstable, the exposure time t_(clean)required to achieve stability. If the system is found to be in a stablestate then sample measurements can be performed. For example as shown instep 740 the state may be considered stable if the difference betweenthe reference sample intensity for two successive measurements (n andn−1) is within a range that is within a desired measurementrepeatability that is associated with a “clean” state. However, as shownin step 745, the state may be considered unstable if the differencebetween the reference sample intensity for two successive measurements(n and n−1) is greater than the range that is within a desiredmeasurement repeatability that is associated with a “clean” state. Ifthe system is found unstable (indicating the cleaning process isincomplete) then the exposure dose required to achieve system stabilitycan be estimated. This exposure dose can be converted to an effectivemeasurement time to which the system (and reference sample) can beexposed as indicated in step 750. The exposure may in one embodiment bemade in a single event, as opposed to exposure through a series ofindividual reference measurements. Following exposure of the system tothe requisite cleaning dose, the series of monitoring measurements onthe reference sample can again be performed as indicated by there-evaluation step 760. The process may be repeated until it isconfirmed that the instrument is in fact in a stable “clean” state. Asshown in the exemplary technique of FIG. 7, a change between twointensity measurements was determined by subtracting one intensitymeasurement from another. However, it will be recognized that adifference or change in two intensity measurements may be identified bya wide range of methods of comparing the two measurements and thereforethe variation between two measurements may be quantified in a wide rangeof manners. For example, ratios may also be utilized to quantify thevariation. Thus, it will be recognized that the measurement data may beanalyzed, compared and quantified with a wide range of mathematicalmethods while still utilizing the concepts described herein. Inaddition, although described with relation to evaluating two successivemeasurements (n and n−1), it will be recognized that the twomeasurements need not be successive but rather merely any twomeasurements may be evaluated to determine a variation from onemeasurement to the another.

It follows that by tracking the stability of the system (as a functionof time, usage, etc.) the details (i.e. reference measurement frequency,effective dose per exposure, trace concentration of oxygen present ininstrumental volume, etc) associated with the cleaning process can beadjusted so as to ensure efficient instrument cleaning and henceenhanced system stability. In this manner the tool can be operated insuch a fashion as to optimize instrument performance.

An example of the potential results from this process are presented asplot 800 in FIG. 8, wherein the normalized intensity from a referencesample at a single VUV wavelength is plotted as a function of effectivedose. As is evident from the figure the intensity from the referencesample increases generally linearly upon exposure for a period of timebefore stabilizing. It follows that the ability of an optical instrumentto perform accurate measurements, prior to completion of such a cleaningprocess, would be significantly compromised if such cleaning was notperformed.

Non-optical surfaces within such instruments must also be considered asthey can serve as sources of contaminants that may be adsorbed on them,and which may be liberated at a later time. Accordingly, it is desirableto manufacture such surfaces in a manner so as to minimize stickingprobabilities for potential contaminants. This may involve specificmachining processes and/or the application of appropriate coatings toensure vacuum compatibility is achieved.

In order to lessen the migration of contaminants from extraneoussurfaces within the instrument to optical elements it is advantageous tooperate the tool in such a manner as to minimize the evaporation rate ofsuch species. At a given temperature, the evaporation rate of moleculeswill increase as the ambient pressure is reduced. Additionally, the meanfree path of these molecules will also increase under such conditions.As a consequence such molecules will exhibit a greater propensity todistribute themselves throughout the available volume, increasing thelikelihood of their encountering optical surfaces and sticking to them.Thus, from a contamination perspective it is desirable to minimize thetime wherein the volume of VUV optical instrumentation is maintained atreduced pressures.

Just as optical elements in VUV metrology tools are susceptible tocontamination effects, so are samples to be measured themselves. Theextent of sample contamination will depend largely on both theenvironment to which samples are exposed to, following their creation,and on the duration of that exposure. Hence, to help achieve accuratemeasurements of sample properties it is desirable to properly accountfor surface contaminant layer(s) which may exist. This is particularlyimportant in cases where the samples under study are comprised ofultra-thin films whose thicknesses are comparable to those of thecontaminant layers themselves. This importance is further underscored insituations where the contaminant layer(s) exhibit a high degree ofabsorbance, relative to the ultra-thin films under study.

The conventional approach to this problem has been to attempt tocompletely remove the contaminate layer from the entire surface of thesample prior to initiating measurements. Typically, whole wafer cleaningis performed using thermal heating, microwave radiation, UV radiation orsome combination of these or other techniques. There are numerousdifficulties associated with such full wafer cleaning methodologies.Such systems are often relatively large and as such likely to beconfigured as stand-alone systems, outside the controlled environment ofa VUV optical metrology instrument. As a result, samples must betransferred between the cleaning system and the metrology instrument,giving rise to the possibility of re-contamination.

Furthermore, while moisture may be readily removed using whole wafercleaning methodologies, complete and uniform removal of othercontaminants may prove problematic. This follows from the difficultiesassociated with generating a power flux possessing sufficient energy,amplitude and spatial uniformity to ensure that contaminants are fullyremoved from all regions of the sample. Residual contaminants followingcleaning may lead to inaccurate measurement results and misleadingconclusions regarding the spatial uniformity of sample properties.

Stand-alone spot-cleaning techniques suffer from shortcomings even incases where they are integrated into the controlled environment of a VUVmetrology instrument. To ensure accurate measurements results areobtained it is may be desirable to precisely align spot-cleaning systemswith optical metrology instrumentation such that cleaning andmeasurement spot locations are coincident.

Accordingly it would be advantageous if spot-cleaning capabilities couldbe integrated in to optical metrology tools in such a manner as to avoidalignment concerns through use of a common optics module. Furthermore,if such capabilities could be incorporated in a means that required veryfew additional components, above and beyond those already present in theoptical metrology tool, system design and cost requirement could begreatly reduced.

A novel means of accomplishing this is to utilize the measurementradiation itself to both clean and characterize the sample. Hence,discrete locations on samples can be cleaned via exposure to measurementradiation immediately preceding measurement. In addition to eliminatingcleaning/measurement alignment concerns entirely, this approach avoidsneedlessly “treating” the vast bulk of the sample surface area. Thetechniques provided herein may still be utilized however through the useof separate light sources utilized as a cleaning light source and ameasurement light source. One, both or neither of such light sources maybe a VUV light source.

A further benefit of this technique is that it offers the ability toreadily measure and characterize the contaminant layer itself. Themanner in which this can be accomplished is illustrated in the flowchart900 of FIG. 9. By obtaining an optical response from a contaminatedsample before and after removal of the contamination layer and thenanalyzing the results it is possible to determine the properties(thickness, optical properties, composition, roughness, etc.) of thecontaminant layer. With this knowledge in hand, data can then becollected from other contaminated locations on the sample and analyzedin order to characterize the properties of the sample. In other words,once the properties of the contaminant layer on a given sample aredetermined it is in principle, possible to accurately characterize otherlocations on the sample without first cleaning them. In addition to theobvious throughput advantages, owing to the decrease in combinedclean/measurement cycle time, this technique can also provide valuableinformation regarding the contaminant layer itself which may be used torefine sample processing methodologies.

As shown in FIG. 9 step 910, reflectance data may first be collectedfrom a first location of a “dirty” sample which has a contaminant filmpresent. Next at step 920 the first location on the sample is cleaned byexposing the location to measurement radiation. At step 930 “clean” datamay then be collected from the first location following the removal ofcontaminate film in step 920. At step 940 the clean data from the firstlocation of the sample may be analyzed to determine the properties ofthe sample. At step 950 the measured properties of the sample from step940 may be utilized to analyze the dirty data from the first location(the data from step 910) in order to determine properties of thecontaminant film. Next at step 960 “dirty” data may be collected fromanother location on the sample with the contaminant film present. Thenin step 970 by using the “dirty” data from the other location obtainedin step 960 and the measured properties of the contaminant film obtainedin step 950, properties of the sample may be determined in step 970 forthe other location without requiring a cleaning step of the otherlocation. Multiple locations may be analyzed in this manner by returningcontrol from step 970 to 960 and repeating the process.

It follows that the method of FIG. 9 could be employed in a mannerwhereby the properties of the contaminant layer are determined at one orsome number of locations on a given sample, or series of samples, andthen used during the analysis of subsequent measurement locations on thesame or different samples, once the validity of the approach has beendemonstrated. Thus, the contaminant layer need not be analyzed at allthe measurement locations. Alternatively, in situations where the natureof the contaminant layer is expected to vary significantly from onelocation to the next, it is possible to clean every measurement locationprior to data collection. Additionally, it is also possible to measureeach location on the sample prior to and immediately following cleaningin order to determine the properties of both the contaminant layer andthe underlying sample.

Of course, it is possible to combine the use of this integratedspot-cleaning methodology with other stand-alone, whole wafer orspot-cleaning techniques. Such an approach may be advantageous insituations where samples are highly contaminated and/or where manylocations on a given sample are to be measured. Under such circumstancesthe combination of cleaning methods may aid to speed the cleaningprocess.

The same sample cleaning methodology outlined herein can also be used toprepare calibration and/or reference samples utilized by opticalmetrology systems to ensure that a high level of measurement accuracy isachieved. Furthermore, the properties of these “cleaned” samples can bemonitored over time to track the “health” of such samples. In thismanner the repair and/or replacement of such samples could be scheduledto coincide with other preventative maintenance activities.

In some instances further insight into the characteristics ofcontaminate layers themselves may be gained through observation of theresponse of such layers to the cleaning process. That is, by recordingthe removal rate of the contaminate film as a function of accumulateddose it may be possible to determine something of its chemical nature.Once correlated with additional analytical data, a library of cleaningresponse profiles could be generated. This library could then bereferred to during subsequent measurements wherein unknown contaminantswere to be characterized.

As an example, FIG. 10 presents exemplary cleaning response profiles forthree different contaminate layers 1000, 1010, and 1020. The exemplarycleaning response profiles illustrate the profile variations that may beseen from different contaminate layers. With these profiles stored in acleaning response library, profiles from unknown contaminant films couldbe categorized through comparison. The profiles could be compareddirectly or could be fit using a parameterized model, wherein theresultant parameter values could be used to distinguish betweencontaminant species.

Thus as described above, a property of a sample layer may be changed byexposure of the sample to optical radiation and the changes may becharacterized through the use of measurements performed before and afterthe exposure to optical radiation. In the example described above thechange may comprise removing a contaminant layer from the sample.However, it will be recognized that other changes in the sample may becharacterized. Thus, for example, a contaminant layer need not bepresent but rather the some other layer or portion of the sample may becharacterized. In one embodiment, a property of a layer or portion ofthe sample that is to be analyzed through use of the optical metrologytool may be characterized. In such an embodiment, the layer or portionmay remain after the exposure to optical radiation however some state ofthe layer or portion may be changed. By characterizing the changes thatoccur, information about the original properties of the layer or portionmay be obtained.

For example, exposure to the optical radiation may change the bondstructure, species concentrations, or other physical properties of thesample. In such examples, the original bond structures, speciesconcentrations/migration or other physical properties may bequantifiable based upon the amount of change detected with some dose ofoptical radiation. Thus, how the layer reacts to the optical radiationmay render useful information about the original properties of thelayer. The changes in the layer may be analyzed through quantifiedmeasurements of the change detected, alternatively through comparison toknown optical response profiles (such as stored in a response profilelibrary) or other techniques.

In one example, a silicon oxide film that contains nitrogen may becharacterized through such techniques. For example, VUV optical exposuremay preferentially impact nitrogen contained in such film by causingnitrogen migration and/or changing the bond structure holding thenitrogen in such films. Detected optical response variations before andafter the optical radiation exposure may thus provide useful informationwith regard to the original status of the nitrogen bonds (tightlybonded, loosely bonded, etc.), the nitrogen concentration, or the like.It will thus be recognized that in a general form the techniquesprovided herein provide for detection of characteristics of a samplethrough the analysis of a film before and after a property of the filmhas been changed due to exposure to optical radiation.

In order to help achieve accurate and repeatable results from opticalmetrology instruments operating in the VUV, the environmental monitoringmethodology of FIG. 1, the contaminant monitoring methodology of FIG. 7and the sample cleaning methodology of FIG. 9 may be combined to formthe underlying operation basis for such equipment.

Example of a VUV optical metrology instrument well suited to benefitfrom use of the methods herein described are disclosed in U.S.application Ser. No. 10/668,642, filed on Sep. 23, 2003, now U.S. Pat.No. 7,067,818 and U.S. application Ser. No. 10/909,126, filed on Jul.30, 2004, now U.S. Pat. No. 7,126,131 the disclosures of which are bothexpressly incorporated herein by reference. The metrology instrument maybe a broad-band reflectometer specifically designed to operate over abroad range of wavelengths, including the VUV.

An example of such an instrument 1100 is presented in FIG. 11. As isevident the source 1110, beam conditioning module 1120, optics (notshown), spectrometer 1130 and detector 1140 are contained within anenvironmentally controlled instrument (or optics) chamber 1102. Thesample 1150, additional optics 1160, motorized stage/sample chuck 1170(with optional integrated desorber capabilities) and reference sample1155 are housed in a separate environmentally controlled sample chamber1104 so as to enable loading and unloading of samples withoutcontaminating the quality of the instrument chamber environment. Theinstrument and sample chambers are connected via a controllable couplingmechanism 1106 which permits the transfer of photons, and if so desiredthe exchange of gases to occur. Both the instrument chamber 1102 andsample chamber 1104 are connected to vacuum and purge sub-system 1175that is complete with appropriate vacuum connections 1176, valves, purgeconnections 1177 and pressure gauges 1178 such that environmentalcontrol can be independently exercised in each chamber. In this manner,the environmental vacuum and backfill techniques described above may beaccomplished upon each chamber independently or together. Thus, thevacuum and backfill techniques (one or both) may be performed upon theinstrumentation/optics chamber, the sample chamber and/or both chambers.

Additionally a processor (not shown) located outside the controlledenvironments may be used to coordinate and facilitate the automatedmonitoring methodologies and to analyze the measured data. It isrecognized that the processor may be any of a wide variety of computingmeans that may provide suitable data processing and/or storage of thedata collected.

While not explicitly shown in FIG. 11 it is noted that the system couldbe equipped with a robot and other associated mechanized components toaid in the loading and unloading of samples in an automated fashion,thereby further increasing measurement throughput. Further, as is knownin the art load lock chambers may also be utilized in conjunction withthe sample chamber to improve environmental control and increase thesystem throughput for interchanging samples.

In operation light from the source 1110 is modified, by way of the beamconditioning module 1120 and directed via delivery optics through thecoupling mechanism 1106 and into the sample chamber 1104, where it isfocused onto the sample 1150 by focusing optics 1160. Light reflectedfrom the sample 1150 is collected by the focusing optics 1160 andre-directed out through the coupling mechanism 1106 where it isdispersed by the spectrometer 1130 and recorded by the detector 1140.The entire optical path of the device is maintained within controlledenvironments which function to remove absorbing species and permittransmission of VUV photons.

A more detailed schematic of the optical aspects of the instrument ispresented in FIG. 12. The instrument is configured to collect referencedbroad band reflectance data in the VUV and two additional spectralregions. In operation light from these three spectral regions may beobtained in either a parallel or serial manner. When operated in aserial fashion reflectance data from the VUV is first obtained andreferenced, following which, reflectance data from the second and thenthird regions is collected and referenced. Once all three data sets arerecorded they are spliced together to form a single broad band spectrum.In parallel operation reflectance data from all three regions arecollected, referenced and recorded simultaneously prior to datasplicing.

The instrument is separated into two environmentally controlledchambers, the instrument chamber 1102 and the sample chamber 1104. Theinstrument chamber 1102 houses most of the system optics and is notexposed to the atmosphere on a regular basis. The sample chamber 1104houses the sample and sample and reference optics, and is openedregularly to facilitate changing samples. For example, the instrumentchamber 1102 may include mirrors M-1, M-2, M-3, and M4. Flip-in mirrorsFM-1 and FM-3 may be utilized to selective chose which light source1201, 1202 and 1203 is utilized (each having a different spectralregion). Flip-in mirrors FM-2 and FM-4 may be utilized to selectivechose one of spectrometers 1204, 1216, and 1214 (again depending uponthe chosen spectral region). Mirrors M-6, M-7, M-8 and M-9 may beutilized to help direct the light beams as shown. Windows W-1 and W-2couple light between the instrument chamber 1102 and sample chamber1104. Windows W-3, W4, W-5 and W-6 couple light into and out of theinstrument chamber 1102. Beam splitter BS and shutters S-1 and S-2 areutilized to selectively direct light to a sample 1206 or a reference1207 with the assistance of mirrors M-2 and M4 as shown (the referencemay be a mirror in one embodiment). The sample beam passes throughcompensator plate CP. The compensator plate CP is included to eliminatethe phase difference that would occur between the sample and referencepaths resulting from the fact that light traveling in the sample channelpasses through the beam splitter substrate but once, while lighttraveling in the reference channel passes through the beam splittersubstrate three times due to the nature of operation of a beam splitter.Hence, the compensator plate may be constructed of the same material andis of the same thickness as the beam splitter. This ensures that lighttraveling through the sample channel also passes through the same totalthickness of beam splitter substrate material.

When operated in a serial fashion VUV data is first obtained byswitching the second spectral region flip-in source mirror FM-1 andthird spectral region flip-in source mirror FM-2 into the “out” positionso as to allow light from the VUV source to be collected, collimated andredirected towards beam splitter element BS by the focusing mirror M-1.Light striking the beam splitter is divided into two components, thesample beam 1255 and the reference beam 1265, using a near-balancedMichelson interferometer arrangement. The sample beam is reflected fromthe beam splitter BS and travels through the compensator plate CP,sample shutter S-1 and sample window W-1 into the sample chamber 1104,where it is redirected and focused onto the sample 1206 via a focusingmirror M-2. The reference shutter S-2 is closed during this time. Thesample window W-1 is constructed of a material that is sufficientlytransparent to VUV wavelengths so as to maintain high opticalthroughput.

Light reflected from the sample is collected, collimated and redirectedby the sample mirror M-2 back through the sample window, where it passesthrough the sample shutter and compensator plate. The light thencontinues on unhampered by the first spectral region flip-in detectormirror FM-2 and the second spectral region flip-in detector mirror FM-4(switched to the “out” position), where it is redirected and focusedonto the entrance slit of the VUV spectrometer 1214 by the focusingmirror M-3. At this point light from the sample beam is dispersed by theVUV spectrometer and recorded by its associated detector.

Following collection of the sample beam, the reference beam is measured.This is accomplished by closing the sample shutter S-1 and opening thereference shutter S-2. This enables the reference beam to travel throughthe beam splitter BS, reference shutter S-2 and reference window W-2into the sample chamber 1104, wherein it is redirected and focused bymirror M-4 onto the plane reference mirror 1207 which serves as thereference. The reference window is also constructed of a material thatis sufficiently transparent to VUV wavelengths so as to maintain highoptical throughput.

Light reflected from the surface of the plane reference mirror 1207travels back towards the focusing reference mirror M-4 where it iscollected, collimated and redirected through the reference window W-2and the reference shutter S-2 towards the beam splitter BS. Light isthen reflected by the beam splitter towards the focusing mirror M-3where it is redirected and focused onto the entrance slit of the VUVspectrometer 1214.

The path length of the reference beam 1265 is specifically designed soas to match that of the sample beam 1255 in each of the environmentallycontrolled chambers. It follows that the quality of the controlledenvironments of the instrument can be assessed by monitoring theintensity from the reference arm as described earlier in FIG. 1. Asdescribed above with reference to FIG. 7, the amount of opticalradiation that various optical elements of the system are exposed to mayimpact the amount of surface contaminates that may be present upon thevarious optical elements. Thus, to help further balance the referencepath that the reference beam follows and the sample path that the samplepath follows it may be desirable to balance the optical radiation dosethat each path is exposed to. In this manner, in addition to thebalancing of the optical path lengths and elements, the contaminatesrelated to the optical elements may also be relatively balanced betweeneach path. In addition, the techniques for determining the contaminationstate of the optical path described herein may be separately performedupon each path to monitor the state of each of the paths.

Following measurement of the VUV data set, the second spectral regiondata set is obtained in a similar manner. During collection of thesecond region spectral data both the second spectral region sourceflip-in mirror FM-1 and the second spectral region detector flip-inmirror FM-2 are switched to the “in” position. As a result, light fromthe VUV source 1201 is blocked and light from the second spectral regionsource 1203 is allowed to pass through window W-3, after it iscollected, collimated and redirected by its focusing mirror M-6.Similarly, switching the second spectral region detector flip-in mirrorFM-2 into the “in” position directs light from the sample beam (when thesample shutter is open and the reference shutter is closed) andreference beam (when the reference shutter is open and the sampleshutter is closed) through the associated window W-6 and onto the mirrorM-9 which focuses the light onto the entrance slit of the secondspectral region spectrometer 1216, where it is dispersed and collectedby its detector.

Data from the third spectral region is collected in a similar fashion byflipping “in” the third spectral region source flip-in mirror FM-3 andthe third spectral region detector flip-in mirror FM-4, while flipping“out” the second spectral region source flip-in mirror FM-1 and thesecond spectral region detector flip-in mirror FM-2.

Once the sample and reference measurements for each of the spectralregions have been performed a processor (not shown) can be used tocalculate the referenced reflectance spectra in each of the threeregions. Finally, these individual reflectance spectra are combined togenerate a single reflectance spectrum encompassing the three spectralregions.

When operated in a parallel mode, the source and detector flip-inmirrors are replaced with appropriate beam splitters so that data fromall three spectral regions are recorded simultaneously.

Just as supplementary light sources could readily be added to thereflectometer of FIG. 12, specific VUV sources could also be integratedwith the common optical module for the purposes of system and/or samplecleaning in situations where it is deemed desirable. For example, in onealternative a supplementary VUV light source may be utilized for systemand/or sample cleaning. Such a supplementary VUV light source may be alight source that is of a higher intensity then the primary VUV lightsource. Such a supplementary VUV source may also be a single wavelengthline source or have other wavelength characteristics that differ fromthe primary VUV light source. Use of a high intensity light source mayimprove the cleaning throughput. In one embodiment relating to the useof a supplementary VUV light source, it may be desirable to configuresuch source such that light from the source is directed towards mirrorM-1 via flip-in mirrors, shutters or the like (not shown). This wouldallow cleaning of the mirror M-1. In such a configuration thesupplemental light source would encounter most if not all of theelements of the optical path of the primary VUV light source utilizedfor making sample measurements. It will be recognized that the benefitsof the techniques disclosed herein may be achieved without encounteringall of elements of the optical path of the primary VUV light sourcealthough it may be desirable to encounter a substantial number of suchelements. Further, it will be recognized that if a supplementary VUVlight source is utilized, it will be desirable to couple such lightsource to the system in a manner that the path of such light iscontained in an environmentally controlled optical path.

The systems of FIG. 11 and FIG. 12 may be utilized as stand-alone toolsor may be integrated with another process tool. In one embodiment, thesystems of FIG. 11 and FIG. 12 may be merely attached to a process toolwith some mechanism that allows for transport of the sample between theprocess tool and the metrology tool sample chamber. In anotheralternative, the sample chamber may be constructed in a manner that itshared within the process tool such that the metrology tool and theprocess tool may be more tightly integrated together. For example theinstrumentation/optics chamber may communicate with a sample chamberthat is formed with a process tool through the use of a window, gatevalve or other coupling mechanism. In this manner the sample need notneed not leave the environment of the process tool, rather the samplemay be contained within a region of the process tool such as aprocessing chamber, a transport region or other region within theprocess tool.

As is evident the system presented in FIG. 11 and FIG. 12 containsexemplary components to facilitate the environmental monitoringmethodology outlined in FIG. 1, the system contaminant monitoringmethodology outlined in FIG. 7 and the contaminated sample measurementmethodology presented in FIG. 9. In general all three effects (i.e.accumulation of absorbing species within the instrumental volume,contamination of optical surfaces within the instrument and samplecontamination) can significantly influence optical data in the VUV andhence, it may be desirable to simultaneously employ all threemethodologies in order to achieve optimum system performance frominstruments operating in this spectral region. The operational flowchart1300 presented in FIG. 13 provides an example of how this may beaccomplished.

First the volume of the instrument is evacuated to a pre-determinedpressure at step 1305. Next, the instrument is backfilled with anon-absorbing gas to a pre-determined measurement pressure 1310. Oncethe measurement pressure has been attained the system is prepared formeasurement using the system contaminant monitoring methodology outlinedin FIG. 7 as indicated at step 1315. Thus, at step 1315 the varioussystem contaminant steps 710-760 of FIG. 7 may be performed. Once thestate of the system is deemed stable and “clean” at step 1315, theintensity spectrum from the reference sample is immediately obtained atstep 1320 for time t₁. The concentrations of absorbing species arepresumed to be at their lowest achievable level at this point in time.The intensity spectrum obtained at this time may be utilized as part ofthe environmental monitoring process corresponding to step 130 ofFIG. 1. Measurements on test samples may then be performed for somepre-determined time period at step 1325 utilizing the contaminatedsample measurement methodology outlined in FIG. 9 steps 910-970.

Once the pre-determined measurement time has elapsed, the state of thecontrolled environment is assessed again at step 1330 by preparing thesystem for measurement utilizing the system contaminant monitoringmethodology outlined in FIG. 7. Performing the system contaminantmonitoring and cleaning steps again may place the various opticalsurfaces in a similar state as they were at step 1315. This allows thecompletion of the environmental monitoring steps under a condition thatmore accurately matches the system condition for the first intensitymeasurement recorded at step 1320. Thus, after step 1330, the intensityspectrum from the reference sample is again recorded at time t₂ asindicated at step 1335. The ratio of the two intensity spectra (time t₁and time t₂) from the reference sample is then calculated and analyzedin steps 1340 and 1345 in order to determine the concentrations ofabsorbing species N₁, N₂, etc. At step 1350, the determinedconcentrations are then compared to threshold values in order todetermine whether or not the environment in the instrument is suitableto support further measurements. If the environment is deemed suitable(i.e. the determined concentrations are less than the threshold values),then further measurements may be conducted once the system is againprepared for measurement via the process of FIG. 7 as indicated by step1355. Conversely, if as indicated by step 1360 the environment is foundto be inadequate to support further measurements the environment may beregenerated by reinitiating the procedure of FIG. 13 by returningcontrol to step 1305. Thus, steps 1335-1360 correspond to theenvironmental monitoring steps 150-195 of FIG. 1.

As shown in FIG. 13, the various concepts of the environmentalmonitoring and regeneration of FIG. 1, the system contaminant monitoringand cleaning of FIG. 7 and the sample cleaning of FIG. 9 may all beintegrated together for use in controlling an optical metrology tool. Itwill be recognized that the ordering of the various techniques may bechanged and that FIG. 13 is merely illustrative of one way of combiningthe various concepts. For example, the concepts may be implementedserially rather than integrating the steps together as shown in thetechnique of FIG. 13. Moreover, the concepts need not all be utilizedtogether. For example, in alternative embodiments only one or two of theconcepts may be utilized.

The extent to which contamination effects (both environmental andsurface/sample) will degrade the performance of VUV optical metrologyinstrumentation will generally depend on a wide range of factorsincluding, but not limited to, tool design, method of operation,measurement frequency, sample load methodology, and samplecharacteristics. As a result, it is expected that the operationalprocedure outlined in FIG. 13 may be modified on a case by case basis inorder to ensure optimum instrument performance is maintained.

Further modifications and alternative embodiments of this invention willbe apparent to those skilled in the art in view of this description.Accordingly, this description is to be construed as illustrative onlyand is for the purpose of teaching those skilled in the art the mannerof carrying out the invention. It is to be understood that the forms ofthe invention herein shown and described are to be taken as presentlypreferred embodiments. Equivalent elements may be substituted for thoseillustrated and describe herein and certain features of the inventionmay be utilized independently of the use of other features, all as wouldbe apparent to one skilled in the art after having the benefit of thisdescription of the invention.

1. A method of monitoring surface contaminant levels on optical elementsof an optical metrology tool comprising: performing a plurality ofintensity measurements; analyzing the intensity measurements of at leasttwo of said plurality of measurements; determining the stability of thesurface contaminant levels of the optical elements from a comparison ofintensity measurements of the at least two of said plurality ofmeasurements, wherein the optical elements are part of the metrologytool and are utilized as part of a optical path providing light to orfrom a surface to be measured with the optical metrology tool.
 2. Themethod of claim 1, wherein the state of the surface contaminant levelsis determined to be stable if the comparison is within a desired range.3. The method of claim 1, wherein the state of the surface contaminantlevels is determined to be unstable if the comparison is not within adesired range.
 4. The method of claim 3, further comprising performing acleaning operation to clean the surfaces of at least some of the opticalelements after the state of the surface contaminant levels is determinedto be unstable.
 5. The method of claim 4, wherein the cleaning operationcomprises exposing the optical surfaces to radiation.
 6. The method ofclaim 5, wherein the exposure to optical radiation occurs in thepresence of a gas.
 7. The method of claim 6, wherein the gas is presentin an environment that is at a sub-atmospheric pressure.
 8. The methodof claim 6, wherein the gas accelerates the cleaning operation.
 9. Themethod of claim 5, wherein the radiation comprises light having at leastsome wavelengths less than deep ultra-violet (DUV) wavelengths.
 10. Themethod of claim 1, wherein the optical metrology tool comprises areference optical path and a sample optical path.
 11. The method ofclaim 10, wherein the intensity measurements are performed utilizing thereference optical path.
 12. The method of claim 10, wherein theintensity measurements are performed utilizing the sample optical path.13. The method of claim 10, wherein the reference optical path opticallymatches a sample optical path.
 14. The method of claim 13, whereinradiation exposure doses are balanced between the reference optical pathand the sample optical path.
 15. The method of claim 1, furthercomprising performing a first evacuation and backfill operation of atleast one environmentally controlled chamber of the optical metrologytool.
 16. The method of claim 15, further comprising adjusting theenvironment of the optical metrology tool if the environmentalcontamination state of the optical metrology tool is determined to beunsuitable for further use.
 17. The method of claim 16, wherein theadjusting the environment comprises performing a second evacuation andbackfill operation of at least one environmentally controlled chamber ofthe optical metrology tool.
 18. A method of monitoring surfacecontaminant levels on optical elements of an optical metrology tool thatutilizes at least some wavelengths less than deep ultra-violet (DUV)wavelengths, the method comprising: providing a reference optical pathand a sample optical path, the reference optical path and the sampleoptical path being optically balanced, the optical elements being partof the reference optical path, the sample path or both and are utilizedin providing light to or from a surface to be measured with the opticalmetrology tool; performing a plurality of intensity measurements on thesurface utilizing at least some wavelengths less than DUV wavelengths;analyzing the intensity measurements of at least two of said pluralityof measurements; determining the stability of the surface contaminantlevels based at least on part on the analysis of the intensitymeasurements of the at least two of said plurality of measurements,wherein the intensity measurements are performed utilizing at least oneof the reference optical path or the sample optical path.
 19. The methodof claim 18, wherein the intensity measurements are performed utilizingthe reference optical path.
 20. The method of claim 18, wherein theintensity measurements are performed utilizing the sample optical path.21. The method of claim 18, wherein the stability of the surfacecontaminant levels of the both the reference optical path and the sampleoptical path are determined.
 22. The method of claim 21, wherein asimilar radiation exposure dose is provided to both the referenceoptical path and the sample optical path.
 23. The method of claim 18,wherein a similar radiation exposure dose is provided to both thereference optical path and the sample optical path.
 24. The method ofclaim 18, wherein the analyzing the intensity measurements of at leasttwo of said plurality of measurements comprises performing a comparisonof said at least two of said plurality of measurements.
 25. The methodof claim 24, wherein the state of the surface contaminant levels isdetermined to be stable if the comparison is within a desired range. 26.The method of claim 24, wherein the state of the surface contaminantlevels is determined to be unstable if the comparison is not within adesired range.
 27. The method of claim 24, wherein the comparisonidentifies a variation between said at least two of said plurality ofmeasurements.
 28. The method of claim 24, further comprising performinga cleaning operation to clean the surfaces of at least some of theoptical elements after the state of the surface contaminant levels isdetermined to be unstable.
 29. The method of claim 28, wherein thecleaning operation comprises exposing the optical surfaces to radiation.30. The method of claim 29, wherein the exposure to optical radiationoccurs in the presence of a gas.
 31. The method of claim 30, wherein thegas is present in an environment that is at a sub-atmospheric pressure.32. The method of claim 30, wherein the gas accelerates the cleaningoperation.
 33. A method of cleaning surface contaminants on opticalelements of an optical metrology tool comprising: performing a pluralityof intensity measurements of a first surface, the optical elementsproviding being part of an optical path to or from the first surface,wherein the intensity measurements expose the optical elements toradiation under conditions suitable for removing the surfacecontaminants; analyzing the measured intensity of at least two of saidplurality of measurements; determining whether surface cleaning of theoptical elements is desirable based upon the analyzing of the measuredintensities; wherein if surface cleaning is determined to be desirable,surface cleaning is performed by exposing the optical elements toadditional radiation.
 34. The method of claim 33, wherein the state ofthe surface contaminants is determined to be acceptable if an intensityvariation between the at least two of said plurality of measurements iswithin a desired range.
 35. The method of claim 33, wherein surfacecleaning of the surface contaminant levels is determined to be desirableif intensity variation between the at least two of said plurality ofmeasurements is not within a desired range.
 36. The method of claim 33,wherein the radiation includes wavelengths comprising at least DUVwavelengths.
 37. The method of claim 36, wherein at least a plurality ofoptical components utilized for performing the surface cleaning are alsoutilized for performing optical metrology sample measurements.
 38. Themethod of claim 37, wherein a light source for performing opticalmetrology sample measurements is also utilized as a light source for theradiation utilized to clean the optical elements.
 39. The method ofclaim 36, wherein the optical metrology tool comprises a referenceoptical path and a sample optical path.
 40. The method of claim 39,wherein the reference optical path optically matches a sample opticalpath.
 41. The method of claim 33, wherein the exposure to additionalradiation occurs in the presence of a gas.
 42. The method of claim 41,wherein the gas is present in an environment that is at asub-atmospheric pressure.
 43. The method of claim 41, wherein the gasaccelerates the cleaning operation.
 44. The method of claim 6, whereinthe gas comprises oxygen.
 45. The method of claim 7, wherein the gascomprises oxygen.
 46. The method of claim 8, wherein the gas comprisesoxygen.
 47. The method of claim 30, wherein the gas comprises oxygen.48. The method of claim 31, wherein the gas comprises oxygen.
 49. Themethod of claim 32, wherein the gas comprises oxygen.
 50. The method ofclaim 41, wherein the gas comprises oxygen.
 51. The method of claim 42,wherein the gas comprises oxygen.
 52. The method of claim 43, whereinthe gas comprises oxygen.